This invention relates to localization and tracking systems for mobile robots having particular application to inspection of large, three-dimensional structures such as liquefied petroleum gas tanks.
Liquefied Natural-petroleum Gas (LNG) and Liquefied Petroleum Gas (LPG) are often stored in vast tanks measuring up to 60 m in height and 100 m in diameter. During normal operation, they are cooled to −162° to keep the contained gas liquefied. These tanks must be inspected and maintained regularly by humans, resulting in extended warm-up times for reaching adequate temperatures. Accelerating the shutdown period is indispensable for lowering the cost of such operations and can be achieved by using mobile robot platforms, which are sent into the tanks at much lower temperatures. These robots are equipped with inspection sensors and are proposed to replace the need for humans in such a hazardous environment.
Localization and tracking of the mobile robots inside the tank is necessary to ensure comprehensive inspection. Mud forms at the bottom of the tanks and can cause the robot's wheels to slip. This makes tracking techniques solely based on integration methods, such as wheel encoders, unreliable. An absolute measurement system is mandatory, and while wireless radio frequency techniques like the Global Positioning System (GPS) are not allowed by regulations, lasers offer a possible solution. One option is to point with the laser in front of the robot and monitor the reflected light with a camera, letting the robot chase the laser spot. Alternatively, a lightsensitive sensor array can be placed on top of the robot. A coordinate system can be formed by placing its origin at the midpoint of the sensor array. The local distance between laser point and origin of the coordinate system can form an error signal for tracking and control purposes. The first method has the disadvantage of directing the laser into the mud, which can absorb the light and make it difficult to track. Light sensitive sensors, commonly made out of semi-conducting devices, are expensive at larger scales.